(1) Field of the Invention
This invention relates to a niobium powder used for a capacitor having excellent leakage current characteristics; a sintered body produced from the niobium powder; and a capacitor having the sintered body.
(2) Description of the Related Art
Capacitors to be incorporated in electronic apparatuses such as portable phones and personal computers are demanded to have a small size and a high capacitance. Among such capacitors, a tantalum capacitor has been widely used, in view of high capacitance relative to its size, and excellent performance. Generally, in the tantalum capacitor, a sintered body of tantalum powder is used as a positive electrode, and therefore in order to increase the capacitance of the capacitor, the weight of the sintered body must be increased.
When the weight of the sintered body is increased, the capacitor necessarily becomes larger in size and fails to satisfy the demand for a small-sized capacitor. In order to solve this problem, a capacitor containing a sintered body of a powdery material having a dielectric constant higher than that of tantalum has been studied. Niobium and titanium are mentioned as examples of the powdery material having a high dielectric constant.
However, a sintered body of the above-described material has a high specific leakage current index. Since niobium and titanium have high dielectric constants, a capacitor having high capacitance can be produced from these materials, but lower specific leakage current index is required in order to produce a capacitor of high reliability. Specific leakage current index, i.e., leakage current per unit capacitance, can be used to evaluate whether high capacitance can be obtained while maintaining leakage current at a practically permissible low level.
The specific leakage current index is determined as follows. A sintered body having a dielectric layer formed thereon by electrolytic oxidation is prepared, and 70% of formation voltage is continuously applied to the sintered body for three minutes. The leakage current during the application of voltage is divided by the product of formation voltage during electrolytic oxidation and capacitance of the sintered body. Thus, the specific leakage current index is expressed by the following formula:
specific leakage current index=[LC/(Cxc3x97V)]
wherein LC: leakage current, C: capacitance and V: formation voltage.
In the case of a sintered body of a tantalum powder, a specific leakage current index is not more than 1,500 [pA/(xcexcFxc2x7V)], as calculated from capacitance and leakage current described in a catalogue entitled xe2x80x9cCAPACITOR GRADE TANTALUMxe2x80x9d by Showa Cabot Supermetals K.K. In order to guarantee this value, it is generally accepted that the actual measured value of specific leakage current index must be at most ⅓ to xc2xc of the value calculated from the catalogue, and a preferred leak current index is not more than 400 [pA/(xcexcFxc2x7V)]. However, a conventional sintered body of niobium or titanium powder has a specific leakage current index much higher than the preferred leak current index, and thus a capacitor containing the sintered body of niobium or titanium has poor reliability and is impractical for use.
In view of the foregoing, an object of the present invention is to provide a niobium powder suitable for the production of a capacitor having a low specific leakage current index.
Another object of the present invention is to provide a sintered body of niobium powder, used for a capacitor having a low specific leakage current index.
A further object of the present invention is to provide a capacitor with a low specific leakage current index, which has an electrode composed of a sintered body of niobium powder.
In a first aspect of the present invention, there is provided a niobium powder having a degree of nitridation represented by a nitrogen content (hereinafter referred to as xe2x80x9cnitrogen contentxe2x80x9d) of at least about 500 ppm by weight and not more than about 7,000 ppm by weight and having a mean particle diameter of at least about 0.2 xcexcm and smaller than about 3 xcexcm.
In a second aspect of the present invention, there is provided a sintered body produced from a niobium powder, which exhibits a specific leakage current index of not more than about 400 [pA/(xcexcFxc2x7V)].
In a third aspect of the present invention, there is provided a sintered body produced from the niobium powder concerned with the first aspect of the present invention.
In a fourth aspect of the present invention, there is provided a capacitor comprising the capacitor concerned with the second or third aspect of the present invention, as one electrode, a dielectric formed on the sintered body, and the other electrode.
The niobium powder of the present invention is characterized as having a nitrogen content of at least about 500 ppm by weight and not more than about 7,000 ppm by weight and having a mean particle diameter of at least about 0.2 xcexcm and smaller than about 3 xcexcm. A capacitor produced from the niobium powder exhibits a very low specific leakage current index.
The reason for which the capacitor of the niobium powder exhibits a very low specific leakage current index is inferred below.
Generally, capacitance of a capacitor is represented by the following formula:
C=xcex5xc3x97(S/d)
wherein C: capacitance, xcex5: dielectric constant, S: specific surface area and d: distance between electrodes.
In the above expression, since d=kxc3x97V (k: constant and V: formation voltage), C is represented by the following formula:
C=xcex5xc3x97[S/(kxc3x97V)], and thus, Cxc3x97V=(xcex5/k)xc3x97S.
When specific leakage current index is defined by the following formula as hereinbefore mentioned.
specific leakage current index=[LC/(Cxc3x97V)]
(LC: leakage current), the specific leakage current index [LC/(Cxc3x97V)] can be expressed by the following formula:
specific leakage current index=LC/[(xcex5/k)xc3x97S].
In consideration of the above formulas, in order to decrease specific leakage current index, there may be selected any measure from among decreasing leakage current (LC), increasing (Cxc3x97V), increasing xcex5, and increasing S.
In the present invention, the niobium powder of the present invention has a mean particle diameter of smaller than about 3 xcexcm, the specific surface area of the powder is large. Consequently, the (Cxc3x97V) value, which is the denominator in the above-described formula providing specific leakage current index, is large. However, when the mean particle diameter of the niobium powder is smaller than about 0.2 xcexcm, a sintered body produced from the niobium powder has a problem such that permeation of a negative electrode material into the sintered body becomes difficult. As a result, capacitance of the produced capacitor cannot be increased to the desired extent, and the (Cxc3x97V) value cannot be made large, so that the sintered body is unsuitable for practical use.
Meanwhile, niobium may be bonded more strongly with oxygen than may tantalum, and thus, oxygen atoms in an electrolytic-oxidized film formed on niobium tend to diffuse toward the interior metal, i.e., niobium. In contrast, in the sintered body according to the present invention, a niobium powder is partially bonded with nitrogen, and thus oxygen in an electrolytic-oxidized film formed on niobium is hardly bonded with niobium, preventing diffusion of oxygen atoms toward the niobium. Consequently, the oxidized film can be stabilized and leakage current (LC) may be decreased.
In addition, since the niobium powder according to the present invention comprises a nitrogen content of at least about 500 ppm by weight and not more than about 7,000 ppm by weight, leakage current, serving as the numerator of the above-described formula, becomes especially low. Therefore, specific leakage current index of the sintered body according to the present invention may become particularly low.
As is described above, the sintered body of the present invention has a satisfactory specific leakage current index as low as not more than about 400 [pA/(xcexcFxc2x7V)]. Furthermore, in the present invention, when a nitrogen content in the niobium powder and the mean particle diameter of the niobium powder are optimized, the specific leakage current index may become not more than about 200 [pA/(xcexcFxc2x7V)].
A niobium powder having a mean particle diameter of at least about 0.2 xcexcm and less than about 3 xcexcm serves as a raw material for forming the sintered body. In order to decrease the specific leakage current index, the mean particle diameter is more preferably at least about 0.5 xcexcm and less than about 2 xcexcm. If the mean particle diameter is less than about 0.2 xcexcm, when a capacitor is fabricated from a sintered body produced from the niobium powder, a negative electrode material described below becomes difficult to soak into the sintered body because pores in the sintered body become very small. In contrast, if the mean particle diameter is about 3 xcexcm or larger, the sintered body having a desirable specific leakage current index is difficult to obtain. As used herein, in the case of the niobium powder, the term xe2x80x9cmean particle diameterxe2x80x9d refers to D50 value, i.e., particle diameter value having a cumulative weight % of 50, which is measured by a particle size distribution measurement apparatus (commercial name, Microtrac).
The niobium powder having the above-described mean particle diameter can be produced by means of, for example, pulverization of a sodium-reduced compound of potassium fluoroniobate, pulverization of a hydrogenated niobium ingot followed by dehydrogenation, or carbon-reduction of niobium oxide. The mean particle diameter of niobium powder can be controlled, for example, by the degree of hydrogenation of a niobium ingot, the pulverization time, and pulverization apparatus, when the niobium powder is obtained by pulverization of hydrogenated niobium ingot followed by dehydrogenation.
The thus-obtained niobium powder may contain impurities attributed to the raw material, the reducing agent, and the apparatus employed. Typical impurities are elements M, which include iron, nickel, cobalt, silicon, sodium, potassium, and magnesium. The above-described niobium powder may be washed with an alkali and at least one acid selected from hydrofluoric acid, nitric acid, sulfuric acid and hydrochloric acid. Alternatively, the niobium powder may be washed with the above acid, an alkali, and aqueous hydrogen peroxide. These reagents may be used sequentially or in combination, so as to wash the niobium powder repeatedly for removal of impurities. More specifically, the niobium powder may be sufficiently washed, for example, with sulfuric acid, and residual sulfuric acid may be neutralized by use of an alkali, after which, the niobium powder may be repeatedly washed with water. When nitric acid is used together with hydrogen peroxide for washing the niobium powder, oxidation of the powder by nitric acid can be advantageously prevented. The niobium powder may also be washed by means of another method; for example, the niobium powder is stirred in the above-described reagents for an appropriate period of time, i.e., until the impurity content reaches a predetermined value or less, and the powder is separated from the reagent with stirring.
In the present invention, impurity content of the niobium powder should preferably be reduced as low as possible. Generally, impurity content on the surface of a powder increases in accordance with the surface area, and therefore, in the above-described formula for calculating xe2x80x9cspecific leakage current index,xe2x80x9d xe2x80x9cleakage current (LC)xe2x80x9d serving as the numerator tends to become larger than xe2x80x9c(Cxc3x97V)xe2x80x9d serving as the denominator. However, in the present invention, by suppression of impurity content, an increase of xe2x80x9c(Cxc3x97V)xe2x80x9d serving as the denominator can become larger in relation to xe2x80x9cleakage current (LC)xe2x80x9d serving as the numerator, as compared with typical cases.
When the niobium powder containing the element M as an impurity is used for producing a capacitor, the element M may migrate into a dielectric layer. Therefore, when voltage is applied to the capacitor, the element M may cause abnormal accumulation of electric charge, and specific leakage current index of the capacitor may become larger.
The amount of each of the elements M should preferably be not more than about 100 ppm by weight, or the total amount of the elements M should be not more than about 350 ppm by weight. By reducing the impurity content, a baneful influence on the above-described dielectric layer can be reduced. In order to decrease the specific leakage current index further, the amount of each of the elements M is preferably not more than about 70 ppm by weight, and more preferably not more than about 30 ppm by weight. Also, in order to decrease the specific leakage current index further, the total amount of the elements M is preferably not more than about 300 ppm by weight, and more preferably not more than about 200 ppm by weight.
The niobium powder according to the present invention has the above-described mean particle diameter, and a nitrogen content of at least about 500 ppm and not more than about 7,000 ppm by weight. In order to further reduce the specific leakage current index, the nitrogen content is preferably at least about 1,000 ppm and not more than about 3,000 ppm by weight. When the content is less than about 500 ppm by weight or in excess of about 7,000 ppm by weight, a sintered body having the desired specific leakage current index becomes difficult to obtain. As used herein, nitrogen content refers not to the amount of nitrogen adsorbed onto the niobium powder, but to the amount of nitrogen which has been chemically bound to niobium.
Liquid nitrogen, nitrogen ions, and nitrogen gas may be used as the nitrogen source for nitridation of the niobium powder, and these may be used either alone or as combination of two or more thereof. The niobium powder is preferably subjected to nitridation under a nitrogen gas atmosphere, since a convenient apparatus can be used with easy operation. For example, the niobium powder is allowed to stand under a nitrogen atmosphere, to thereby give a nitrided niobium powder. In this case, the niobium powder is allowed to stand under a nitrogen atmosphere at a temperature of not higher than about 2,000xc2x0 C. for within several tens of hours, to thereby obtain the niobium powder having the intended nitrogen content. Conducting the nitridation at higher temperature may shorten the time for treatment.
In consideration of lower specific leakage current index, in order to obtain the niobium powder having a nitrogen content in the range of about 500 ppm by weight to about 7,000 ppm by weight, after the particle diameter of niobium powder is measured, nitridation temperature and time can be controlled in relation to the particle size under conditions which are determined by a pre-test.
The procedure by which the niobium powder is sintered is not particularly limited, and the conventional procedure can be employed. For example, the niobium powder is press molded into a predetermined shape, and then maintained at a temperature of about 500xc2x0 C. to about 2,000xc2x0 C. under a reduced pressure of about 1 Torr to about 1xc3x9710xe2x88x926 Torr for several minutes to several hours to give the sintered body.
A capacitor containing the above-described sintered body serving as one electrode, the other electrode, and a dielectric sandwiched by these electrodes may be produced. A preferable example of the dielectric of the capacitor includes niobium oxide. For example, the niobium sintered body serving as one electrode is subjected to electrochemical formation in an electrolytic solution, to thereby form a dielectric composed of niobium oxide on a surface of the sintered body. The electrochemical formation in the electrolytic solution is typically performed by using an aqueous solution of a protic acid, for example, an about 0.1% aqueous solution of phosphoric acid or sulfuric acid. When a niobium oxide dielectric is formed by electrochemical formation of the niobium sintered body in the electrolytic solution, the capacitor according to the present invention serves as an electrolytic capacitor and the niobium sintered body becomes a positive electrode.
No particular limitation is imposed on the material of the other electrode of the capacitor according to the present invention. For example, there may be used at least one compound selected from electrolytic solutions, organic semiconductors and inorganic semiconductors, all of which are publicly known in the aluminum electrolytic capacitor industry. As specific examples of the electrolytic solution, there can be mentioned a mixed solution of dimethylformamide and ethylene glycol having incorporated therein 5% by weight of isobutyltripropylammonium borotetrafluoride, and a mixed solution of propylene carbonate and ethylene glycol having incorporated therein 7% by weight of tetraethylammonium borotetrafluoride. As specific examples of the organic semiconductor, there can be mentioned an organic semiconductor containing benzopyrroline tetramer and chrolanyl, an organic semiconductor predominantly comprising tetrathiotetracene, an organic semiconductor predominantly comprising tetracyanoquinodimethane, and an organic semiconductor predominantly comprising a conductive polymer, which is a polymer represented by the following formula (1) or (2) and has been doped with a dopant. As specific examples of the inorganic semiconductor, there can be mentioned an inorganic semiconductor predominantly comprising lead dioxide or manganese dioxide, and a inorganic semiconductor comprising triiron tetroxide. These semiconductors may be used either alone or as combination of two or more thereof. 
In formulas (1) and (2), R1 to R4 independently represent a hydrogen atom, a C1-C6 alkyl group or a C1-C6 alkoxy group; X represents an oxygen, sulfur or nitrogen atom; R5, which is present only when X is a nitrogen atom, represents a hydrogen atom or a C1-C6 alkyl group; and R1 may be bonded together with R2, and R3 may be bonded together with R4, to form a cyclic structure.
As specific examples of the polymers of formula (1) and (2), there can be mentioned polyaniline, polyoxyphenylene, polyphenylene sulfide, polythiophene, polyfuran, polypyrrole, polymethylpyrrole, and derivatives thereof.
When, as the above-described organic semiconductor and inorganic semiconductor, those having a conductivity of about 10xe2x88x922 Sxc2x7cmxe2x88x921 to about 103 Sxc2x7cmxe2x88x921 is used, the impedance of the produced capacitor further decreases and the capacitance of the capacitor may be further increased at high frequency.
When the other electrode is in solid form, carbon paste and silver paste may be successively formed on the other electrode and encapsulated with a material such as an epoxy resin, to thereby fabricate a capacitor. The capacitor may have a niobium or tantalum lead, which has been sintered together with the niobium sintered body or welded to the sintered body after sintering. When the other electrode is in liquid form, the capacitor comprising the above-described electrodes and the dielectric may be placed in a can which is electrically connected to the other electrode for the fabrication of a capacitor. In this case, the electrode of the niobium sintered body is led to the outside via the above-described niobium or tantalum lead, and the lead is insulated from the can by a material such as insulating rubber.
As is described above, a sintered body having low specific leakage current index, obtained according to the present invention, may be used for producing a capacitor having low leakage current and high reliability.
The present invention will now be described in more detail by the following working examples.
Characteristics of a niobium powder, a sintered body and a capacitor were determined by the following methods.
(1) Nitrogen Content in Niobium Powder
The amount of nitrogen bound in a nitrided niobium powder was determined based on thermal conductivity of the powder as measured by an oxygen- and nitrogen-measuring apparatus (supplied by LECO). The ratio (unit: ppm by weight) of the amount of bound nitrogen to the separately measured weight of the nitrided powder was regarded the nitrogen content.
(2) Capacitance of Sintered Body
The capacitance of a sintered body was measured by an LCR meter (supplied by HP), which was connected between the sintered body immersed in aqueous 30% sulfuric acid and a tantalum electrode placed in a sulfuric acid solution. The measurement of the capacitance was conducted at 120 Hz and at room temperature.
(3) Leakage Current (LC) of Sintered Body
The leakage current (LC) of a sintered body was measured as follows. DC voltage equivalent to 70% of the electrochemical forming voltage employed during preparation of a dielectric was imposed for three minutes at room temperature between the sintered body immersed in an aqueous 20% solution of phosphoric acid and an electrode placed in an aqueous solution of phosphoric acid. After voltage application, the current was measured as the leakage current of the sintered body. In the present invention, the imposed voltage was 14 V.
(4) Capacitance of Capacitor
A capacitor was formed into a chip, and the capacitance of the chip was measured by an LCR meter (supplied by HP) which was connected between terminals of the chip at room temperature and at 120 Hz.
(5) Leakage Current of Capacitor
The leakage current of a capacitor formed into a chip was measured as follows. DC voltage equivalent to approximately ⅓ to xc2xc of the electrochemical forming voltage employed during preparation of a dielectric was selected from rating voltages such as 2.5 V, 4 V, 6.3 V, 10 V, 16 V and 25 V. The selected voltage was applied at room temperature between terminals of the produced chip for one minute. After voltage application, the current was measured as the leakage current of the capacitor formed into a chip. In the present invention, the applied voltage was 6.3 V.